A co-cathedral is a cathedral church which shares the function of being a bishop's seat, or cathedra, with another cathedral, often in another city (usually a former see, anchor city of the metropolitan area or the civil capital). Instances of this occurred in England before the Protestant Reformation in the dioceses of 'Bath and Wells', and of 'Coventry and Lichfield'. These two dioceses were each named for both cities that served as bishop's seats.

As of March 2020, the Catholic Church had 322 co-cathedrals, mainly in Europe (140 in Italy alone).

Many are former cathedrals, but even if still in use, those often are not granted co-cathedral status.

Often the diocese with one or more co-cathedrals also has a multiple ("hyphenated") name reflecting these, but some have a co-cathedral not mentioned in the title while other former see titles may also be preserved without having a co-cathedral. Sometimes the first-named city does not have the main cathedral (actual see) but boasts another distinction such as being a national capital or having an august ecclesiastical past.

In Albania, the Roman Catholic Archdiocese of Tirana-Durrës has a co-cathedral in Durrës, Saint Lucia co-cathedral.

In Belgium, the cathedral of the primatial Roman Catholic Archdiocese of Mechelen-Brussels is the Metropolitan St. Rumbold's Cathedral in Mechelen (Malines), the archiepiscopal seat. Its co-cathedral is the Cathedral of St. Michael and St. Gudula in Brussels, the national capital. A third, larger church in Koekelberg (also in Brussels) has the status of minor basilica, without co-cathedral rank, yet it has received papal visits including a papal beatification.

The Roman Catholic Diocese of Sofia and Plovdiv has, besides the Cathedral of St Louis in Plovdiv, a new co-cathedral of St. Joseph in Sofia.

The Roman Catholic Diocese of Ostrava-Opava has a co-cathedral of Our Lady of the Assumption Co-Cathedral in Ostravian-Opavian diocese in Opava besides the Ostrava Savior cathedral.